Modified cellulose ethers, obtainable by reaction of cellulose ethers carrying free hydroxy groups with di- and/or polycarboxylic acids and the use of catalysts, and method for producing the same

ABSTRACT

The invention relates to a method for the production of cellulose ethers, whereby cellulose ethers having free hydroxyl groups are reacted with dicarboxylic and/or polycarboxylic acids and a nitrogen-containing compound. The process further comprises intensively mixing essentially dry, pulverulent cellulose ether with a mixture of organic bifunctional and/or polyfunctional acid and nitrogen-containing compound in a non-nucleophilic organic solvent prior to reacting the cellulose ether to provide the modified cellulose ether which can be stirred into water at a pH greater than or equal to 11 without agglutination.

Modified cellulose ethers which can be obtained by reacting celluloseethers having free hydroxyl groups with dicarboxylic and/orpolycarboxylic acids in the added presence of catalysts, and a processfor preparing them

The present invention is described in the German priority applicationNo. 100 23540.9, filed May 13th, 2000, which is hereby incorporated byreference as is fully disclosed herein.

The present application relates to modified cellulose ethers which canbe obtained by reacting common cellulose ethers having free hydroxylgroups with at least one organic dicarboxylic and/or polycarboxylic acidwhile activating the organic acid(s) with carbodiimides orcarbonyldiimidazoles, and to a process for preparing them. Theresulting, modified cellulose ethers are distinguished by a superior,agglomeration-free ability to be stirred in and a delay in starting toswell when being stirred into aqueous solutions, even at stronglyalkaline pH values (pH≧11).

The preparation of cellulose ethers having the same or differentsubstituents has been disclosed (see, for example, Ullmann'sEnzyklopädie der Technischen Chemie [Ullmann's Encyclopedia ofIndustrial Chemistry], vol. 9, “Cellulose ethers”, Verlag Chemie,Weinheim, 4th edition 1975, pp. 192ff; K. Engelskirchen:“Polysaccharid-Derivate [Polysaccharide derivatives]” in Houben Weyl,vol. E20/III, 4th edtn., Georg Thieme Verlag Stuttgart, 1987, pp.2042ff).

In order to prepare these cellulose ethers, for example methylcellulose, ethyl cellulose, carboxymethyl cellulose, hydroxyethylcellulose, hydroxypropyl cellulose, methylhydroxyethyl cellulose,methylhydroxypropyl cellulose and ethylhydroxyethyl cellulose, thestarting material, i.e. the cellulose, is first of all ground in orderto increase the surface area, with the intention being that the particlesize should as a rule be less than 2.5 mm, and if at all possible evenless than 1 mm. The resulting, voluminous cellulose powder is convertedinto “alkali cellulose” by adding base, such as NaOH, KOH, LiOH and/orNH₄OH, in solid or liquid form. This is then followed, with or withoutthe alkali cellulose being isolated, by a single-step or multistep,continuous or discontinuous etherification using the correspondingreagents. The resulting cellulose ethers are freed, in a known manner,from reaction byproducts using water or suitable solvent mixtures andthen dried, ground and, where appropriate, mixed with other components.

Despite these cellulose ethers having good solubility in cold water, itis frequently a problem to prepare aqueous solutions of these compounds.This is the case, in particular, when the cellulose ether is present asa fine powder having an increased surface area. When such a celluloseether powder comes into contact with water, the individual particlesswell and agglutinate to form relatively large agglomerates whosesurface is thickened in a gel-like manner. However, a certainproportion, which depends on the mixing intensity, of completelyunwetted cellulose ether is present in the interior of theseagglomerates. Depending on the viscosity of the resulting solution, andthe average polymer chain length, it can take up to 24 hours to dissolvethese agglomerates completely.

In order to diminish the clumping which occurs when preparing aqueoussolutions of cellulose ethers, the cellulose ethers can be treated withsurfactants, as described, for example, in U.S. Pat. No. 2,647,064 andU.S. Pat. No. 2,720,464.

In addition to this, it is desirable, for some applications, to have acertain open time, lasting from a few seconds up to several hours. Opentime, or else delay in starting to swell (DSS) means that, after thecomponents, including the cellulose ether, have been mixed, a certainfurther period of time passes before the cellulose ether, if at allpossible abruptly, increases the viscosity of the mixture.

The combination of preventing the cellulose ether clumping and of havingan open time is as a rule achieved by crosslinking cellulose ethers. Inthis connection, crosslinking means the linking of at least twodifferent polymer chains by way of bifunctional or polyfunctionalmolecules, such as dialdehydes, such as glyoxal, glutaraldehyde orstructurally related compounds, and also diesters, dicarboxylic acids,dicarboxamides and an hydrides.

Reacting free hydroxyl groups of the cellulose ether with aldehydes,with the formation of hemiacetals, generates a partial, reversiblecrosslinking, which is cleaved, with a time delay, on dissolving inneutral or weakly acidic water. An abrupt increase in viscosity, withoutclumping, takes place after the powder has been distributed in theaqueous medium and after a defined open time, which can be regulated, byway of the degree of crosslinking, by the quantity of crosslinkingreagent which is added.

CA-C-947 281 describes crosslinking at acid pH using phosphoric acid anddialdehydes, while U.S. Pat. No. 3,372,156 describes crosslinking usingdialdehyde sugars. The mechanism of crosslinking with differentdialdehydes when crosslinking hydroxypropyl cellulose is described indetail by S. Suto and M. Yoshinaka in Journal of Material Science 28(1993), pp. 4644 to 4650.

A feature possessed in common by the abovementioned crosslinkedcellulose ethers, in particular those which crosslink with hemiacetalformation, is that, in an alkaline medium at pH>9, the crosslinkingsometimes opens so rapidly that clumping of the material to be dissolvedoccurs irrespective of the quantities of crosslinking reagent which areused for the crosslinking reaction. As a result, it is no longerpossible to ensure a uniform, rapid development of viscosity at thesought-after point in time. However, if the bonds, by way of which thecrosslinking is brought about, are stabilized, for example by reactingwith propane dihalide or epihalo-hydrin, the resulting crosslinking isso stable that cleavage into the discrete polymer chains no longer takesplace and the crosslinked cellulose ethers in general prove to beinsoluble in water.

U.S. Pat. No. 3,461,115 describes the crosslinking of hydroxyethylcellulose using dicarboxylic acids and the corresponding esters andsalts, resulting in products which can be stirred, withoutagglutination, into water which is at neutral pH. However, thedifficulty of achieving such a crosslinking increases as the number offree hydroxyl groups which are available for a crosslinking decreases,i.e. as the degree of alkylation of the cellulose ether increases.

The problem of the chronologically limited stability of the crosslinkingunder alkaline conditions was partially remedied by developing specialmethods which can be used for preparing products which exhibit a delayin starting to swell, and can be stirred in without agglutination, evenin alkaline media.

These methods include, in particular, the method described in U.S. Pat.No. 1,465,934, in which dialdehydes are combined with boric acid orwater-soluble borates, and the combination of glyoxal solutions andpotassium dihydrogen phosphate (Kongop Hwahak (1999), 10(4), pp. 581 to585).

In addition to this, it is also possible to use silicon-containingreagents for crosslinkings which are partially alkali-stable. An exampleof this is provided by JP 08/183 802, in which alkoxylated andacyloxyalkylated silicon compounds are used.

However, a feature possessed in common by these methods is that they areonly partially effective, or not effective at all, in strongly alkalinemedia, at pH values of >11, as can exist, for example, in buildingmaterial mixtures, since these crosslinkings, too, are rapidly openedunder these conditions.

As described in WO 80/00842, carbodiimides have been used for convertingcarboxymethyl cellulose in aqueous solution. Gel hydrates of highviscosity are formed by hydrophobizing the acid groups of thecarboxymethyl cellulose, by using from 0.5 to 2 equivalents ofcarbodiimide per acid group of the carboxymethyl cellulose.

It is likewise possible to hydrophobize hydroxyethyl cellulose films byreacting with carbodiimides, dissolved in chlorinated hydrocarbons,while catalyzing with strong mineral acids such as phosphoric acid,hydrochloric acid or tetrafluoroboric acid, and also sodium alkoxide, asdescribed in CS-A-174425.

The alcoholate groups of the hydroxyethyl cellulose are blocked byreaction with the carbodiimides, with the formation of urea derivatives.

The object of the present invention was therefore to develop modifiedcellulose ethers which are distinguished by properties which areimproved from the application technology point of view such that thecellulose ethers can be stirred into water at a pH of ≧11 withoutagglutination and ensure a certain delay in starting to swell, whichdelay is in the range of from seconds to hours, even in alkaline medium.

The object is achieved by means of modified cellulose ethers which canbe obtained by reacting cellulose ethers having free hydroxyl groupswith organic, bifunctional and/or polyfunctional carboxylic acids whichhave been activated by reaction with nitrogen-containing compounds, andalso by a process for preparing these modified cellulose ethers, inwhich process cellulose ethers having free hydroxyl groups are reactedwith organic, bifunctional and/or polyfunctional carboxyiic acids whichhave been activated by reaction with nitrogen-containing compounds.

In the present invention, carboxylic acids having more than two acidgroups per molecule are described as being polyfunctional carboxylicacids.

It has been found, surprisingly, that the modified cellulose ethers canbe stirred into aqueous solutions without agglutination and also possessa delay in starting to swell of at least 2 seconds even in an alkalinemedium at a pH of ≧11.

The nitrogen-containing reagents are preferably carbodiimidegroup-containing and/or carbonyldiimidazole group-containing compoundsor their salts.

It is also possible to use reagents which possess more than onecarbodiimide group or carbonyldiimidazole group per molecule.

The choice of the carbodiimide group-containing and/orcarbonyldiimidazole group-containing compounds is in no way restricted.

However, preference is given to using compounds such asdicyclohexyl-carbodiimide or diisopropylcarbodiimide or related, alkylgroup-carrying or aryl group-carrying carbodiimides, including thosewhich are asymmetrically substituted, as carbodiimide group-containingcompounds.

It is likewise possible to use salts of carbodiimide group-containingcompounds, such as N,N′-dicyclohexylcarbodiimide methiodide orN-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride orcomparable compounds.

The quantity of the nitrogen-containing reagent which is used, based onthe cellulose ether, is preferably in the range from 0.01 to 20% byweight. Particular preference is given to using between 0.1 and 5% byweight.

The ratio by weight of the quantity of nitrogen-containing reagent usedto the quantity of organic acid used is preferably from 5:95 to 95:5,with this ratio particularly preferably being from 1.5:1 to 5:1.

The quantity of organic carboxylic acid which is used, based on thecellulose ether, is preferably less than 20% by weight, and isparticularly preferably in the range from 0.2 to 5% by weight.

Preference is given to using compounds such as maleic acid or itsderivatives in which at least one C—H bond is partially or completelyreplaced with a C—C bond as organic dicarboxylic acids.

Particular preference is given to using polycarboxylic acids such ascitric acid or its derivatives in which at least one C—H bond isreplaced with a C—C bond.

When citric acid is used, a more or less pronounced state of markedlyincreased viscosity, which is visible macroscopically as gel formation,is passed through in the course of the process of dissolving in alkalinemedia.

It is likewise possible to use salts of the corresponding carboxylicacids. The cellulose ethers which are preferably employed are methylcellulose, ethyl cellulose, carboxymethyl cellulose, hydroxyethylcellulose and hydroxypropyl cellulose and also their mixed ethershydroxyethylethyl cellulose, hydroxyethylmethyl cellulose andhydroxypropylmethyl cellulose, and also other conceivable mixed ether,including those with additional substituents.

The crosslinking preferably takes place at temperatures in the rangefrom 0 to 150° C., in each case depending on the boiling point of theorganic suspending agent employed, but particularly preferably in therange from 15 to 100° C.

In a preferred embodiment, the commercial quality cellulose ether issuspended in an organic suspending agent, without going into solution,with the water content of the mixture composed of cellulose ether,organic suspending agent and organic acid preferably being less than 20%by weight, based on the quantity of cellulose ether employed.

Particular preference is given to carrying out the process at a totalwater content of less than 10% by weight, in particular at less than 5%by weight.

Compounds which do not react with carbodiimides or carbonyldiimidazoles,in particular acetone, diethyl ether and ethers having alkyl chainscontaining up to 8 carbon atoms per chain, as well as cyclic ethers,such as dihydropyran, dihydrofuran, tetrahydrofuran or dioxane, ethyleneglycol dimethyl ether, diethylene glycol dimethyl ether, triethyleneglycol dimethyl ether, tetraethylene glycol dimethyl ether,straight-chain and branched hydrocarbons having up to 12 carbon atoms,and also cyclic compounds, such as cyclopentane or cyclohexane, oraromatic compounds, such as toluene or benzene or alkyl-substitutedtoluenes or benzenes, are preferably selected as organic suspendingagents.

It is also conceivable to bring essentially dry, pulverulent celluloseethers into contact, by means of intensive mixing, for example in acustomary mixing unit, with a solution comprising a mixture ofbifunctional and/or poly-functional organic carboxylic acid andnitrogen-containing reagent in a nonnucleophilic organic solvent.

The above-described reaction of cellulose ethers with bifunctionaland/or polyfunctional organic carboxylic acids, which are activated byusing nitrogen-containing reagents, such as carbodiimides orcarbonyldiimidazoles, leads to cellulose ethers which can even bestirred into alkaline media without clumping before a perceptibledevelopment of viscosity sets in after a defined period of delay instarting to swell.

In addition to depending on the nature and quantity of the acidemployed, the length of the delay in starting to swell also depends onthe carbodiimide or carbonyldiimidazole employed and is considerablyinfluenced by the magnitude of the pH of the solution to be prepared(the length of the delay in starting to swell is inversely proportionalto the magnitude of the concentration of alkali). However, depending onthe crosslinking conditions which are selected, periods of delay instarting to swell of from a few seconds up to hours can still beachieved even at high pH values of 13 or more. Under given limitingconditions, the quantity of carboxylic acid which is added can also beused to selectively influence the delay in starting to swell.

Any attempt to exclusively react cellulose ethers having a limitednumber of free primary and, in particular, secondary hydroxyl groupswith dicarboxylic and/or polycarboxylic acids under heterogeneousconditions, without any activation of the carboxylic acid(s) withcarbodiimides or carbonyldiimidazoles, does not lead, despite high acidconcentrations, to products which can be stirred into alkaline mediawithout agglutination or which exhibit a delay in starting to swell.

The same applies to exclusively reacting cellulose ethers withcarbodiimides or carbonylimidazoles without adding dicarboxylic and/orpolycarboxylic acids.

The process according to the invention is described in more detail belowwith the aid of implementation examples without, however, beingrestricted by these examples.

Determining the Viscosity

Viscosities, which are given in mPa s, are determined by using a Hopplerfalling-ball viscosimeter to measure, at 20° C., 1.9% aqueous solutionsof the corresponding cellulose ether, with reference to the dry solidscontent and allowing for the present moisture content of the powder.

Determining the Delay in Starting to Swell

The delay in starting to swell is measured at 20° C. using a Brabenderviscosimeter, with this measurement being subjected to software-assistedanalysis. The data in [BUs], which are obtained in this connection,refer to Brabender viscosity units, which are directly proportional to acorresponding viscosity in mPa s.

The cellulose ethers are reacted, and prepared for a Brabendermeasurement, in accordance with the following process:

EXAMPLES 1 TO 17

100 g of cellulose ether (absolutely dry), with the amount weighed outbeing corrected for a residual moisture content of less than 2%, aresuspended in 750 g of dimethoxyethane at from 50 to 70° C.

The appropriate quantity, as specified in the table, of the carbodiimideemployed, dissolved in approx. 30 g of organic solvent (dimethoxyethaneunless expressly indicated otherwise), is firstly metered in, afterwhich, in that order, the appropriate quantity of acid, whereappropriate in dissolved form, is metered in. The mixture is reacted atfrom 50 to 70° C. for two hours and the resulting suspension is thenfiltered off with suction, in the hot, on a glass frit and subsequentlywashed twice with a little acetone. The resulting product is dried at70° C. and then ground using an Alpine mill fitted with a 180μ strainerbasket insert.

The development of viscosity by the product which has been prepared inthis way is investigated in a Brabender viscosimeter usingsoftware-assisted analysis.

For this, an appropriate quantity of the modified cellulose ether, whichquantity depends on the viscosity to be expected, is suspended in waterwhich has been adjusted to different pH values. The measurement isstarted by adding the cellulose ether and, with a starting viscosity ofapprox. 35±2 BUs (Brabender units), the time at which the viscosity isgreater than twice the starting viscosity (delay in starting to swell,DSS) is determined, as is the time of maximal viscosity development (gelstructure) and the time at which the viscosity in practice correspondsto the effective final viscosity. TABLE 1 Dependence of the delay instarting to swell on the pH at different citric acid and carbodiimideconcentrations Max. Final Ex. % by wt. % by wt. DSS⁵⁾ visc.⁶⁾ visc.⁷⁾No. CE¹⁾ of DCC²⁾ of CA³⁾ pH⁴⁾ [s] [s] [s] 1 A 2.5 0.5 12.5 105   260  740 2 A 2.5 0.5 13.0 50   120   450 3 A 5.0 1.0 12.5 475 1 255 2 365 4 A5.0 1.0 13.0 80   190   750 5 A 6.0 1.2 12.5 260   700 1 705 6 A 6.0 1.213.0 120   290 1 330¹⁾Cellulose ether A: hydroxyethylmethyl cellulose, viscosity of theunmodified material, approx. 3 800 mPa s; 26.5% OCH₃, 5% OC₂H₄²⁾Carbodiimide employed: dicyclohexylcarbodiimide³⁾Acid employed: citric acid⁴⁾pH of the aqueous solution into which the modified cellulose ether isstirred⁵⁾DSS: delay in starting to swell; with a starting viscosity of approx.35 BUs the time at which the viscosity exceeds twice the startingviscosity⁶⁾The time at which the development of the viscosity is maximal⁷⁾The time at which the final viscosity has in practice been reached

TABLE 2 Dependence of the delay in starting to swell on different citricacid concentrations when using a constant quantity of carbodiimide andat a pH of 13.0; variation in the viscosity of the unmodified celluloseethers Max. Final Ex. % by wt. of % by wt. of DSS visc. visc. No. CE¹⁾DCC CA pH [s] [s] [s] 7 A 5.0 0.5 13.0 40 80   320 4 A 5.0 1.0 13.0 80190   755 8 A 5.0 1.5 13.0 140 460 1 130 9 B 5.0 0.5 13.0 80 155   48010 B 5.0 1.0 13.0 155 335   920 11 B 5.0 3.0 13.0 235 800 1665¹⁾Cellulose ether B: hydroxyethylmethyl cellulose, viscosity of theunmodified material, approx. 100 000 mPa s; 27% OCH₃, 5% OC₂H₄

TABLE 3 Dependence of the delay in starting to swell and of the progressof the viscosity on the carbodiimide employed % by Max. Final Ex. % bywt. wt. DSS visc. visc. No. CE of CDI¹⁾ CDI²⁾ of CA pH [s] [s] [s] 12 B1.53 a 1.0 13.0 170 325 1 010 13 B 4.21 b 1.0 13.0 60 295 1 285 14 B2.32 c 1.0 13.0 90 500 1 840 15 B 5.13 d 1.0 13.0 130 265   490¹⁾Percent by weight of the carbodiimide employed, based on the dryweight of the cellulose ether, calculated on a quantity of carbodiimideemployed of 0.0121 mol²⁾Carbodiimide employed:a diisopropylcarbodiimideb N,N′-dicyclohexylcarbodiimide methiodide, dissolved indimethoxyethane/acetone 4:1c N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride,dissolved in dichloromethaned 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimidemetho-p-toluenesulfonate, dissolved in acetone/dichloromethane 1:1

TABLE 4 Delay in starting to swell when using maleic acid Max. Ex. % bywt. of Acid % by wt. DSS visc. No. CE DCC¹⁾ employed of acid pH [s] [s]16 B 2.5 maleic acid 0.96 13.0 20 200¹⁾Carbodiimide employed: dicyclohexylcarbodiimide

TABLE 5 Delay in starting to swell when using an hydroxypropylmethylcellulose Ex. % by wt. of % by wt. of DSS Max. visc. No. CE¹⁾ DCC CA pH[s] [s] 17 C 2.5 1.0 13.0 215 475¹⁾Cellulose ether C: hydroxypropylmethyl cellulose, viscosity of theunmodified material, approx. 10 000 mPa s; 29% OCH₃, 5% OC₃H₆

EXAMPLE 18

100 g of cellulose ether (absolutely dry), with the amount weighed outbeing corrected for a residual moisture content of less than 2%, aresuspended, at room temperature (from 18 to 25° C.), in 750 g ofdimethoxyethane. 2.5 g of dicyclohexylcarbodiimide, dissolved in approx.30 g of organic solvent, are firstly metered in, after which, and inthat order, 1.0 g of citric acid, which is likewise in dissolved form inorganic solvent, is metered in. The mixture is stirred at roomtemperature for two hours and the resulting suspension is then filteredoff with suction and subsequently washed twice with a little acetone.The modified product is dried at 70° C. and then ground using an Alpinemill fitted with a 180μ strainer basket insert.

The ground product is tested for its delay in starting to swell inalkaline medium using the Brabender viscosimeter, as described above(Table 6).

EXAMPLE 19

The procedure is analogous to that described in Example 18 apart fromthe fact that the modified product is not dried at 70° C. but is insteaddried at room temperature (from 18 to 25° C.) by simply being left tostand (Table 6). TABLE 6 Conversion to modified cellulose ethers at roomtemperature (from 18 to 25° C.) Ex. % by wt. of % by wt. of DSS Max.visc. No. CE DCC CA PH [s] [s] 18 B 2.5 1.0 13.0 55 180 19 B 2.5 1.013.0 65 350

EXAMPLE 20

100 g of cellulose ether (absolutely dry), with the amount weighed outhaving been corrected for a residual moisture content of less than 2%,are suspended, at room temperature (70° C.), in 750 g ofdimethoxyethane. 1.96 g (0.0121 mol) of carbonyldiimidazole, in approx.30 g of dimethoxyethane, are first of all metered in, after which, andin that order, 1.0 g of citric acid, in dissolved form in organicsolvent, is metered in. The mixture is stirred at 70° C. for two hoursand the resulting suspension is then filtered off with suction andsubsequently washed twice with a little acetone. The modified product isdried at 70° C. and then ground using an Alpine mill fitted with a 180μstrainer basket insert.

The ground product is tested for its delay in starting to swell inalkaline medium using the Brabender viscosimeter as described above(Table 7). TABLE 7 Conversion to modified cellulose ethers usingcarbonyldiimidazole Ex. % by wt. of % by wt. of DSS Max. visc. No. CECDI CA pH [s] [s] 20 B 2.5 1.0 13.0 235 435

EXAMPLE 21

100 g of cellulose ether (absolutely dry), with the amount weighed outhaving been corrected for a residual moisture content of less than 2%,are intimately mixed in a kneading unit, at room temperature for 15minutes, with a mixture composed of 1.53 g of diisopropylcarbodiimideand 1.0 g of citric acid in approx. 30 g of dimethoxyethane. In order toprepare this mixture, the individual components are in each casedissolved in 15 g of dimethoxyethane and mixed together, and heated toapprox. 35° C., directly before being added to the cellulose ether.

The modified product is heated at 70° C. for 2 hours and then groundusing an Alpine mill fitted with a 180μ strainer basket insert.

The ground product is tested for its delay in starting to swell inalkaline medium using the Brabender viscosimeter as described above(Table 8). TABLE 8 Conversion to modified cellulose ethers using anessentially dry cellulose ether powder (residual moisture content <2%)Ex. % by wt. of % by wt. of DSS Max. visc. No. CE CDI¹⁾ CA pH [s] [s] 21B 1.53 1.0 13.0 185 >3 500²⁾¹⁾Carbodiimide employed: diisopropylcarbodiimide²⁾Very slow development of viscosity; the maximum viscosity is still notcompletely reached after 3 500 seconds

COMPARATIVE EXAMPLE 1

The procedure is as described for Examples 1 to 17 apart from the factthat only 1.0 g of citric acid, dissolved in 30 g of dimethoxyethane, ismetered in; that is, no nitrogen-containing reagent is added.

COMPARATIVE EXAMPLE 2

The procedure is as described for Examples 1 to 17 apart from the factthat only 5.0 g of dicyclohexylcarbodiimide, in 30 g of dimethoxyethane,are metered in. TABLE 9 Delay in starting to swell when using only onecomponent (dicarboxylic/polycarboxylic acid or carbodiimide) ComparativeExample % by wt. % by wt. DSS Final visc. No. CE of DCC of CA pH [s] [s]1 B — 1.0 13 —¹⁾ — 2 B 5.0 — 13 —¹⁾ —¹⁾Immediate clumping

1. A modified cellulose ether obtained by reacting cellulose ethershaving free hydroxyl groups with an organic, bifunctional and/orpolyfunctional carboxylic acid which has been activated by reaction witha nitrogen-containing compound.
 2. A process for preparing modifiedcellulose ethers which comprises reacting cellulose ethers having freehydroxyl groups with an organic, bifunctional and/or polyfunctionalcarboxylic acid which have been activated by reaction with anitrogen-containing compound.
 3. The process as claimed in claim 2,wherein the nitrogen-containing compound is a carbodiimidegroup-containing and/or a carbonyldiimidazole group-containing compoundor a salt thereof.
 4. The process as claimed in claim 2, wherein thenitrogen-containing compound employed, based on the cellulose ether,ranges from 0.01 to 20% by weight.
 5. The process as claimed in claim 2,wherein a ratio by weight of the nitrogen-containing compound to of saidorganic bifunctional and/or polyfunctional carboxylic acid ranges from5:95 to 95:5.
 6. The process as claimed in claim 2, wherein said organicbifunctional and/or polyfunctional carboxylic acid, based on thecellulose ether, is less than 20% by weight.
 7. The process as claimedin claim 2, wherein the organic bifunctional and/or polyfunctionalcarboxylic acid employed is maleic acid or a derivative thereof in whichat least one C—H bond is replaced with a C—C bond.
 8. The process asclaimed in claim 2, wherein the organic bifunctional and/orpolyfunctional carboxylic acid employed is citric acid or a derivativethereof in which at least one C—H bond is replaced with a C—C bond. 9.The process as claimed in claim 2, wherein the cellulose ether isselected from the group consisting of methyl cellulose, ethyl cellulose,carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropylcellulose, hydroxylethylethyl cellulose, hydroxyethylmethyl cellulose,hydroxypropylmethyl cellulose, and mixtures thereof.
 10. The process asclaimed in claim 2, wherein the reacting step takes place at atemperature ranging from 0 to 150° C.
 11. The process as claimed inclaim 2, wherein the reacting step takes place in an organic suspendingagent and water.
 12. The process as claimed in claim 11, wherein thewater comprises less than 20% by weight, based on the cellulose ether.13. The process as claimed in claim 2, further comprising mixingessentially dry, pulverulent cellulose ether with the organicbifunctional and/or polyfunctional carboxylic acid and thenitrogen-containing compound in a non-nucleophilic organic solvent toprovide a preliminary mixture and intensively mixing the preliminarymixture prior to said reacting step.